The present invention relates to a thin film transistor-liquid crystal display (TFT-LCD) device, and more particularly to a TFT-LCD device free of a reduction in TFT characteristic due to leakage current and a reverse tilt phenomenon caused by a horizontal electric field generated between adjacent signal and pixel electrodes depending on the rubbed direction of liquid crystal.
Generally, TFT-LCD devices include a first transparent substrate formed with TFTs and pixel electrodes and a second transparent substrate formed with color filters and common electrodes. A liquid crystal is filled in a space defined between the first transparent substrate and a second transparent substrate. Polarizing plates for linearly polarizing visible rays are attached to opposed surfaces of the transparent substrates, respectively. Rubbed orientation films are disposed between the first transparent substrate and the liquid crystal and between the liquid crystal and the second transparent substrate, respectively, so as to arrange liquid crystal molecules in one direction.
As image signal voltages are selectively applied to pixels in such TFT-LCD devices, the arrangement of liquid crystal molecules is varied depending on the applied voltages, thereby displaying an image.
Examples of such conventional TFT-LCD devices will now be described.
FIG. 1 is a layout view illustrating one pixel region of a conventional TFT-LCD device. As shown in FIG. 1, this TFT-LCD device includes gate signal lines 1 and 1a, and data signal lines 2 and 2a extending in perpendicular to the gate signal lines. The gate and data signal lines are arranged along boundaries between neighboring pixels, respectively. At the pixel region, a transparent electrode 4 is disposed. A thin film transistor 3 is provided between the transparent electrode 4 and a cross area of the gate and data signal lines 1 and 2.
The thin film transistor 3 has a gate constituted by the gate signal line 1, a source constituted by the data signal line 2 and a drain connected to the transparent electrode 4.
On the other hand, FIG. 2 is a layout view illustrating one pixel region of another conventional TFT-LCD device. This TFT-LCD device is similar to the TFT-LCD device of FIG. 1, except that the gate signal line la disposed at the front end of the pixel is partially protruded over the pixel region so that it is overlapped with the transparent electrode 4. In other words, the TFT-LCD device of FIG. 2 includes an accumulation capacitor 6 provided by the overlap between the gate signal line 1a and the transparent electrode 4. This accumulation capacitor 6 serves to prevent the transparent electrode 4 from being varied in potential when the thin film transistor 3 is electrically cut off because of a conversion of a selection signal on the gate signal line 1 into a non-selection signal.
Operation of each of the conventional TFT-LCD devices will now be described.
A voltage or a charge applied to the liquid crystal capacitor formed by the area of the transparent electrode (pixel electrode) is determined by image information. For achieving a transmission of a voltage signal required for the transparent electrode 4, accordingly, a selection signal is applied for a predetermined time through the gate signal line 1.
By this selection signal, the thin film transistor is electrically activated to transmit the image signal voltage applied to the data signal line 2 to the transparent electrode 4. As the image signal voltage is transmitted to the transparent electrode 4, the arrangement of liquid crystal molecules is changed to display an image.
When the selection signal applied to the gate signal line 1 is converted into a non-selection signal after the predetermined time has elapsed, the thin film transistor is electrically cut off. As a result, no image signal voltage is transmitted to the transparent electrode 4. However, the charge accumulated in the transparent electrode 4 is maintained for a frame time until a next selection signal is applied.
For such a non-selection gate signal time interval, the image signal voltage of the transparent electrode 4 is gradually decreased because of leakage current from the thin film transistor 3 and leakage current from the liquid crystal.
This effect, namely, a degradation in image signal caused by the leakage currents from the thin film transistor and the liquid crystal can be reduced by overlapping the gate signal line disposed at the front end of the pixel with the transparent electrode to provide the accumulation capacitor 6 using the gate insulating film as its dielectric layer, as shown in FIG. 2
In the conventional TFT-LCD devices, however, a reverse tilt occurs because horizontal electric fields are present between the transparent electrode and the gate signal line adjacent to the transparent electrode and between the transparent electrode and the data signal line adjacent to the transparent electrode, respectively, depending on the rubbed direction of the orientation film.
In FIGS. 1 and 2, regions where such a reverse tilt occurs are shown and denoted by the reference numeral 5. Since the reverse tilt regions 5 allows light beams to pass therethrough when the image signal is dark, they serve as a factor of degrading the picture quality of the LCD.
This problem may be solved by a photo-shield metal film formed on an electrode substrate opposed to the transparent electrode 4 and adapted to photo-shield the reverse tilt region 5.
However, regions respectively ranged in certain distances A and B from the gate signal line 1a and the data signal line 2a can not be actual effective display areas because the reverse till regions 5 are photo-shielded. In the case of FIG. 2, the pixel may not be driven due to electrical defects caused by pin holes possibly present between the gate signal line 1a and the transparent electrode 4 at the region corresponding to the accumulation capacitor 6. As a result, the total effective display area is reduced, resulting in a degradation in yield.